In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium, to form characters and image. The ink-jet printing methods have been rapidly spread because of their various advantages such as easiness of full coloration, low costs, capability of using ordinary paper as the recording medium, non-contact with printed images and characters, etc.
Among such printing methods, in view of enhancing the weather resistance and water resistance of printed images and characters, an ink-jet printing method utilizing an ink containing a pigment as the colorant has now come to dominate (for example, refer to PCT pamphlet WO 00/39226, JP 10-87768A and JP 2004-2662A).
PCT pamphlet WO 00/39226 discloses a water-based ink containing a pigment-containing vinyl polymer in which a graft polymer obtained from a macromer is used as the vinyl polymer to achieve a high print density.
In JP 10-87768A, there is disclosed a water dispersion containing an insoluble colorant and a graft copolymer dispersant having a main chain and at least one side chain in which both the chains are respectively produced from an ethylenically unsaturated monomer, and one of the main chain and side chain constitutes a hydrophilic moiety and the other constitutes a hydrophobic moiety containing at least one monomer selected from the group consisting of aryl esters of acrylic acid, aryl esters of methacrylic acid, N-aryl acrylamides, N-aryl methacrylamides and vinyl aryl esters in an amount of at least 50% by weight based on the total weight of the hydrophobic moiety.
In JP 2004-2662A, there is disclosed the use of polypropylene glycol monomethacrylate for enhancing an ejection property and a dispersion stability of the ink.
Although these conventional water-based inks are excellent in print density and dispersion stability, there is such a demand that the water-based inks exhibit a still more excellent performance upon printing on any of ordinary papers and coated papers such as photographic papers and glossy papers.